Rotors, particularly in larger turbo machines, are formed by welding together a plurality of disks. Because of axial temperature gradients experienced during operation of these machines, the disks can be made of different materials at different locations along the rotor axis. For example, in the highest temperature segment of a rotor, where high creep strength is desired, a nickel alloy having a face-centred cubic atomic structure and a high proportion of chromium can be used. Other alloying elements, such as for example molybdenum, tungsten and vanadium, can be used to additionally increase the creep strength. The rotor in this region therefore can include a plurality of nickel alloy disks welded together by a weld made of nickel alloy.
In the low temperature segment where lower cost material with a high yield strength and toughness may be used, the rotor may be made of ferritic-martensitic steel with a body-centred cubic atomic structure. The rotor in this region therefore can include a plurality of steel disks welded together by a weld made of steel.
At the interface barrier between the high and lower temperature regions welds of nickel alloy may be used to weld disks of nickel alloy and steel together.
Particularly in the case of rotors of large steam or gas turbines, the quality of the weld seams between the individual rotor disks can be important to ensuring the mechanical integrity of the rotor. It can therefore be very desirable for the weld to be inspected for defects as accurately as possible and with as high a resolution as possible, using non-destructive inspection methods such as ultrasonic or isotope inspection. This may involve the passing of an inspection signal through an outer surface of the rotor angled towards or through the joining weld.
Ultrasound inspection methods can have significant advantages as compared to isotope methods. However, attenuation of the ultrasonic inspection signal can reduce the resolution of the inspection method, particularly at inspection points of the weld distant from the inspection signal source, rendering the inspection method ineffective. There are a number of causes of attenuation. One cause is high attenuation in some materials, nickel alloys for example. Reflection and refraction is another cause and is most evident when the ultrasonic wave passes through two materials with different atomic structure for example from steel, which has a low attenuation property, across into a weld made of nickel alloy. Further attenuation, although not to the same extent as across atomic structure boundaries, can occur across microstructure interfaces, such as between a nickel alloy disk and nickel weld.
During disk manufacture, to ensure optimum mechanical properties the disk undergoes numerous heat treatment and preparation steps. In contrast, the weld, while it may have the same material composition as that of the disk, does not undergo the same heat treatment steps. As a result a pseudo forged—cast boundary and resulting microstructure difference is created between the disk and weld where weld metal displays the properties of “casting” while the disk the properties of “forging”. As an ultrasonic inspection signal passes from a nickel alloy disk through to a nickel weld, attenuation occurs at the disk/weld microstructure interface.
While inspection methods utilising isotope radiation do not suffer from the same attenuation and reflection/refraction problems of inspection methods utilising ultrasonic waves, it is known to only deviate the angle of the isotope radiation no more than a few degrees from the radial centreline of the weld being inspected. As a result isotope inspection methods are limited in their ability of determine the size and depth of any defects. As the normal process of repairing a defective weld is to remove portions of the weld only up to the defect, imprecise location of the defect necessitates removing more weld than is necessary.